Tellington Touch (or TTouch) has a belly lift, it requires two people, holding a long and narrow towel, to lift, hold for about 10 seconds, then slowly release (release should take at least twice as long as the lift) along the belly starting at the elbows and steadily moving back towards the flank. It helps with gut movement, colic, cinchy-ness, and to raise sway backs.
This exercise may help where the back lifts (fingering the belly) might lack, as a back lift engages the belly muscles to make the horse support its belly, but the belly lifts temporarily take the downward pressure off the back.
The Pelvic Tilt is also recommended if a horse won't lift its back. The PT is a gentle push, hold, and release movement on the point of the butt to help release tension in the back and shoulders. Make the movement forward up to 4 seconds, and slowly release.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.